The anodization of single crystalline silicon wafers in hydrofluoric acid (HF)-ethanol solutions, results in the formation of pores along certain crystal orientations <100>. This form of silicon is known as porous silicon and is typically formed by immersing a single crystalline wafer in an HF-ethanol electrolyte and applying a positive potential. Porous silicon formed by electrochemical etching is usually in the form of a single layer on a single crystalline wafer. Depending on the etching conditions, pore sizes can range from nanometers to tens of micrometers. Porous silicon is classified as either macroporous (pore diameter d>50 nm), mesoporous (2 nm<d<50 nm) or microporous (d<2 nm). Microporous silicon is also referred to as nanoporous silicon. In order to prepare porous silicon powder from porous silicon layers formed on single crystalline wafers, the porous layer must be detached and further agitated to create porous particles. This process has several drawbacks and limitations. During the processes of drying the porous layer and agitation, the silicon structure is likely to crack. Also, the yield of porous silicon (48% porosity) from a 4 inch diameter wafer, created by anodizing for 5 minutes at a growth rate of 1.3 microns/minute, is only 0.7% per gram of starting material. Next, scale-up of this process is difficult because the yield of porous silicon from the process is limited by the diameter of the silicon wafers. In addition, there are many challenges in scaling up electrochemical etching processes to large diameter wafers. Some of these challenges include: the difficulty in achieving a safe and reliable cell design for withstanding extremely corrosive chemicals; making an ohmic contact over the back side of the wafer in order to ensure uniform etching; controlling flow of the electrolyte across the wafer for uniform current density; and the general cost of large diameter silicon wafers. It is for these reasons that there is a need for a low cost and reliable production method capable of producing large quantities of porous silicon particles.
Li et al. (US Patent Application Publication No. US2004/0166319) describe a porous silicon powder comprising individual silicon particles wherein only the outermost layer of each individual particle is porous. In particular, the porous layer has a maximum thickness of only 500 nm (0.5 microns). In making these particles, a stain etch method is employed. A porous silicon powder is subjected to ultrasonic agitation to yield individual silicon nanoparticles. This process allows for using powders instead of single crystalline silicon wafers, enabling etching of a much higher surface area per gram of the material. In addition, this process can produce nanoparticles doped with any type of dopant (n-type, p-type, etc.) if appropriately-doped silicon is used as a precursor material. Examples of various known dopants include arsenic (As), gallium (Ga), phosphorus (P), boron (B), antimony (Sb), erbium (Er), and combinations thereof. Stain-etching is typically performed in an aqueous mixture of hydrofluoric and nitric acids. The reaction process can be described as:Si+2h+→Si2+(hole injection)HNO3+HNO2+H2O2HNO2+2OH−Si2++2OH−→Si(OH)2 Si(OH)2+6HF→H2SiF6+2H2O+H2 The regeneration of HNO2 makes the reaction autocatalytic and the etching rate depends upon the amount of NO2 formed in the reaction:HNO3+HNO22NO2+H2O.Thus, the process is limited by the presence of HNO2 at the surface of the silicon sample. In order to make silicon nanoparticles, the porous silicon powder is ultrasonically agitated in a suitable solvent which causes the porous outermost layers to break up and be dispersed into the solvent. Hence, a mechanical action between the porous silicon layers of the powder and cavitation bubbles induced by ultrasonic agitation result in the generation of individual silicon nanoparticles. However, in all instances, the resulting silicon nanoparticles must be separated from the remnants of the larger silicon particles and the porosity is limited to a maximum thickness of only 500 nm (0.5 microns).
An object of the present invention is to provide silicon nanosponge particles wherein each particle is comprised of a plurality of nanocrystals with pores disposed between the nanocrystals and throughout the entire nanosponge particle.
Another object of the present invention is to provide a method for producing silicon nanosponge particles from a metallurgical grade silicon powder.